rethink hclust tree traversal: it is simple

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegan@2851 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
use hclustMergeOrder in as.hclust.spantree

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegan@2852 edb9625f-4e0d-4859-8d74-9fd3b1da38cb